The present invention relates to a means and methods of controlling the heating of foods in a microwave oven, primarily to achieve differential heating between two or more edible substances positioned adjacent one another in a multi-component food product. It has been found that the desired degree of control by one food and hence the amount of heating of the other food is determined by certain physical properties of the food products.
In the heating of foods in a microwave oven, temperature differentials and differential heating have been achieved through the use of package shielding material. For example, it has long been practiced in microwave cooking to use microwave reflective packaging with various sized apertures to control the amount of microwave radiation simultaneously impinging on various foods. Such reflective packaging was metal or contained metal. The foods desired to be heated were separated in the package with each food having one or more preselected apertures in the shield adjacent the food. The heating of each food was tuned by selecting the appropriate size and/or shape of the aperture to achieve a desired temperature within the heating time of the package. Such packaging was relatively effective for certain food concepts which allowed separation of the components, but was expensive and required empirical design to be effective.
A major drawback to such an approach was that it required that each individual food component be in a separate compartment to separate each of the food substances to achieve the proper heating of each component. Another major drawback was that the shielding material was metal. In years past oven manufacturers objected to and recommended against the use of metal in a microwave oven for fear of possible damage to the oven circuitry and because metal packaging can cause arcing which is a consumer detriment. Further, metal shielding is difficult to use with some packaging materials and is also expensive.
Numerous patents have addressed some of these problems, see for example, U.S. Pat. Nos. 4,081,646, 4,233,325, and 4,122,324.
To date, shielding of foods has been a packaging function and no one has been commercially successful in achieving controlled simultaneous heating of two or more foods absent this type of shielding. Even in the use of two of more food components together, no one has taught how to select properties of the control food to function as a microwave energy transmission inhibitor adjacent to and between the source of microwave energy and another food or how to select properties of one food component to accomplish controlled heating of other food components. Further, even with the above described metal shielding being available it has found little use and most microwave heating is still done until the foods being simultaneously heated reach some temperature with little or no control of the balance of the heating.
Likewise, U.S. Pat. Nos. 2,600,566 to Moffet and 2,714,070 to Welch, when read in context, discuss the need for metallic shielding "packaging" to control heating of multi-component food systems. Further, these particular packaging concepts do not appear to be disposable, but appear to be permanent receptacles. Both patents discuss the concept of differential temperatures, for example, between a sauce and an ice cream through the use of metal shielding. Neither Welch nor Moffet discusses controlled heating of one food with a shielding or control food via control of the thickness, and dielectric properties of the shielding layer. For example, Welch states in columns 3 and 4. "Due to the action of shield 45, energy from the cooker 10 can penetrate the food body only from the top, thereof, through paper member 50. Since the sauce 49 is a relatively high loss substance, it will absorb substantially all the microwave energy passing therethrough with the result that little or no microwave energy passes into the ice cream body 47. The ice cream body 47 thus remains frozen and, due to the ice crystals therein, substantially entirely reflects any energy which does pass thru sauce 49 back into sauce 49. Since the cake member 48 is a relatively good heat insulator, the ice cream body 47 is not melted to any substantial extent by conductive heat from the sauce 49 . . . The resulting product is an ice cream having a hot fudge sauce which may be eaten directly from the paper container 46 inside the shield member 45."
Surprisingly, it has been found that by control of the thickness, and microwave load (a combination of dielectric properties of a control layer and the thermal loads) or by the control of thickness and dielectric constant of a control layer and the thermal loads of each component highly effective cooking control can be achieved. Thermal load can be defined as specific heat in calories/gram multiplied by the mass in grams. Another dimension of thermal performance is thermal diffusivity which is defined as the thermal conductivity (heat input rate in calories/seconds multiplied by distance in centimeters multiplied by temperature difference in degrees centigrade) divided by density and specific heat of the product. It has been found that certain foods can achieve a reflectance of about 65% and a transmission of as low as 20% or even lower, e.g., 10%, for high water layers, of the incident power and can function as an effective shield within given time constraints. Further, it was surprisingly found through calculation that conductive heat transfer from the shielding component to the shielded food component, when made in accordance with the invention, had little practical effect on heating of the shielded component within the short time that is required to heat many foods in a microwave oven, making it possible to allow the shielding layer to heat without detriment to the shielded layer. These factors make it possible to use edible foods or edible substances as shields to control heating of other foods in a microwave environment reducing the need for metal or other nonedible shields and insulating layers. The result is the ability to make new types of microwave foods and food systems that cannot be properly heated together in any other manner.